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Questions tagged [speed-of-light]

The speed of light is a fundamental universal constant that marks the maximum speed at which energy and information can propagate. Its value is $299792458\frac{\mathrm{m}}{\mathrm{s}}$.

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What would happen if $c$ wasn't constant, but the quotient of $c$ with the scale factor $a(t)$ was? [closed]

I'm supposing there are strong reasons to disregard this scenario but I can't understand exactly what would it imply, and why it is not at all considered in cosmology. What I'm talking about is: we ...
Iolanda Navone's user avatar
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What is a simple example that contradicts the theory that light (photons) could have a super small mass? [duplicate]

The obvious example is that, to bring something that has mass to the speed $c$ requires infinite energy due to special relativity. But what if a kid asked me "What if light/photos had a super ...
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Is a photon truly massless? [duplicate]

First of all, I am not a physicist or mathematician, not even a hobbyist but the following statements have always puzzled me: $E=mc^2$ A photon is a desecrated particle of energy. A photon is ...
AUser's user avatar
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Physical meaning of $vx/c^2$ in Lorentz transformation

In Lorentz transformation, this is the formula of time dilation $$ t' = \gamma \left( t - \frac{vx}{c^2} \right) $$ It can be derivated from the formula of length contraction and be proved ...
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Is there a good quantum explanation of refraction? [duplicate]

I'm aware of the classical explanation of refraction which deals with light being a wave that gets "slowed" down while passing from a medium to another. One problem that I have with this ...
PicPuc's user avatar
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Black hole production via light speed [duplicate]

As a test particle with rest mass approaches $c$ relativistic mass increases to infinity, does this mean that the energy required to continue the acceleration will create a black hole (as time also ...
John Pryme's user avatar
2 votes
1 answer
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When you are in a gravitational field, do object far away get physically closer to you as you get closer to the mass?

An observer A is close to a black hole and an observer B one light year away. They are both remaining at constant radial distance from the black hole. A is at 2 Rs away from the center of the black ...
Zach's user avatar
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Second postulate of Special Relativity - Finite Invariante Speed?

@JohnRennie, the most decorated physics stack exchange user of all time, answered the question whether the first postulate of special relativity implies the second (Einstein's first postulate ...
Real Pattern's user avatar
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If the speed of causality changes, could you go FTL?

In the middle of some research, I reached a sort of confusion that I’d like to sort out. In flat space FTL is impossible, because in a Minkowski metric, $$\mathrm{d}s^2=c^2 \mathrm{d}t^2-\mathrm{d}x^2-...
controlgroup's user avatar
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How could I calculate the time it will take for light and mass to go towards a black hole and come back, to and from constant radial distances?

If you have a "perfect mirror" and a "perfect trampoline" at some constant distance outside a black hole's event horizon: a) How would a shell observer at some distance farther ...
Zach's user avatar
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Is it possible to get an electromagnetic wave equation if the speed of light were infinite?

In an old question: How would night sky look like if the speed of light was infinite? the best answer was voted down to negative credits. I cannot understand why. From Maxwell's equations, we derive ...
wiljo's user avatar
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Questions about speed of gravity [duplicate]

If gravity "travels" at $c$, and the sun is travelling "forward", does it mean the planets are actually orbiting various points "behind" the center of the sun? Does it ...
Curious Steve's user avatar
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Beam displacement at focal point

I'm working on a project that uses galvanometers with mirrors. They're configured to have a focal point of 175mm. The motors rotate at 1 mechanical degree per 500mV, that will be 2 degrees optical per ...
j.valerio's user avatar
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The speed of light in vacuum (but which speed?)

Since there are several different definitions of speed describing light propagation like Group velocity, Front velocity and Phase velocity. Which speed is meant when the phrase "The speed of ...
azerbajdzan's user avatar
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Inconvenience of speed of light in optic fiber

As far as I'm concerned, optic fiber is great in order to transport information quickly using light. Since light needs to undergo total internal reflection every single time it hits the walls of the ...
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Plausible explanation for anomalous plasma antenna time difference of arrival?

Consider the below-pictured time difference of arrival system consisting of two plasma antennas* with opposing DC bias. Samples of time difference of a 100MHz signal are taken as the TDOA system is ...
James Bowery's user avatar
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Is the meter relative to time?

Is the meter relative when we are near the speed of light? I was reading a physics book and I found that the meter is the length that light travels for an amount of time, so since time is relative ...
Angel Echavarria's user avatar
1 vote
1 answer
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Understanding the math of the Michelson-Morley experiment

I'm having some conceptual misunderstandings of the Michelson-Morley experiment. The time for the beam going perpendicular ($t_{across}$) of the aether wind I am getting: $$(ct)^{2}=d^{2}+(vt)^{2}\\ (...
Qubit's user avatar
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3 answers
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Why is the speed limit of special relativity $C$ equal to the speed of light $c$ and why must there be only one?

DISCLAIMER: this question is different from other questions posted here previously despite accidentally similar titles. This question is not a duplicate of the one linked bc the linked question is ...
ConformalSymmetry's user avatar
2 votes
3 answers
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Is a Lorentz transformation allowing an infinite value $c$ still a proper Lorentz transformation?

Is it correct to say that inertial systems are related by Lorentz transformations even if we do not know if the "invariant speed" is finite or infinite? To me, this is incorrect because $c$ ...
Real Pattern's user avatar
1 vote
3 answers
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What are relativistic particles? [duplicate]

What are relativistic particles? I got it in a question of mechanics. So, what is it about a particle that makes it "relativistic"?
Nishkarsh Singh's user avatar
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Can the velocity of an image produced by a plane mirror be greater than that of light?

We are currently studying ray optics in school and it made me wonder if the velocity of an image produced by a plane mirror can theoretically be greater than that of light. Using the relation $v_i = 2 ...
Milkprisonersvostok1's user avatar
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Non-homogenous Helmholtz equation in 3+1D: Green's function and solution

I've been reading Jackson's Chapter 8.10 and trying to find the Green's function for a non-homogenous Helmholtz equation. The problem is in cylindrical coordinates. I first made a Fourier transform to ...
Rosabella M's user avatar
7 votes
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Fermat principle: how did they know the speed of light in different mediums?

Pierre de Fermat derived his principle of least time by considering the path that light would take when traveling between two points. He postulated that light would follow the path that minimized the ...
Andy Chow's user avatar
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Proof of the invariance of $c$ using the Lorentz group

Apologies if this question was already asked a few times but i could only find proofs of the invariance of $ ds^2 $. Is there any way of proving the 2nd postulate (that $c$ is invariant in all ...
Tomás's user avatar
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Speed of light along $y$ and $z$ axes when there is relative motion

There are two coordinate systems. One is stationary and the other moves in the positive $x$-direction with a speed of $v$. I don't know how to interpret the $\sqrt{V^2 - v^2}$ term since light always ...
Christina Daniel's user avatar
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Why the photons are deflected during the refraction? [duplicate]

From what I learnt, when photons are passing throw a denser environnement with an positive angle (from the atmosphere to water for exemple), they are slow down. But I can't understand how this ...
Jay Labarsurlakantik 's user avatar
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What is the speed of light during reflection? [duplicate]

What is the speed of light in a vacuum when the light reflects off of a mirror?
Christina Daniel's user avatar
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2 answers
177 views

Rindler coordinates and objects possibly exceeding the speed of light

Assume $c=1$ So I've been on a bit of a journey trying to derive special relativity by myself and in doing so was analyzing the viewpoint of a uniformly accelerating observer. I was particularly ...
NaiDoeShacks's user avatar
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How Taylor series is compatible with special relativity?

In mathematics, an analytic function is defined by its possession of a Taylor series with a positive radius of convergence ($R_c​>0$). Notably, certain analytic functions—such as holomorphic ...
Omid's user avatar
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The speed of an impulse transmitted along a string

A supernova explosion on the far side of the Sun ejects a mass with approximately the same mass of the Sun directly at the back side of the Sun. If this ejected mass is travelling arbitrarily close to ...
Not a physics student's user avatar
11 votes
1 answer
866 views

Northern lights / aurora borealis "pre-warning" - how does this work w.r.t timing and different particle / wave speeds?

There is an article in the newspaper today entitled "Northern lights predicted in US and UK on Monday night in wake of solar storms". I assume that the reason that someone can make this ...
jaimet's user avatar
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3 votes
1 answer
109 views

How is it possible that photons already move at the speed of light the moment they pop into existence? [duplicate]

This is a thought experiment and I might be horribly wrong. If we have an electron-positron annihilation a photon pops into existence. This photon is then supposedly moving at speed of light at the ...
Jurre Groenewegen's user avatar
5 votes
1 answer
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Would the effective speed of an Alcubierre drive be limited by the propagation speed of gravity?

The idea of a warp drive is to "expand space behind the ship and contract it in front"- in this way reaching a target destination faster than one could conventionally. However, the actual ...
elfeiin's user avatar
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Can anyone explain the relationship between the refractive index, the speed, wavelength and angle of a wave?

Can anyone explain the relationship between the refractive index, the speed, wavelength and angle of a wave? in my book is states that $$n = \frac{v_1}{v_2} = \frac{\sin θ_1}{\sin θ_2} = \frac{λ_1}{...
dayum's user avatar
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2 answers
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Why is $c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}$?

I'm sorry if this is a duplicate but I didn't find my answer. I'm currently studying maxwell's equations and I know that by comparing the wave equation for either the magnetic or the electric field \...
Axodarap's user avatar
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Does light accelerate? [duplicate]

If considering the general formula for acceleration, ie. Δ V/ΔT, we would get zero as the velocity of light is constant and does not change. However, what about when photons travel through different ...
Mel's user avatar
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Can we use the fabric of spacetime to go faster than the speed of light?

If the fabric of spacetime isn't bound by the limit of the speed of light (the universe is expanding faster than the speed of light), could humans somehow wrap a spaceship in a bubble of the fabric of ...
Kellan Heerdegen's user avatar
1 vote
2 answers
74 views

On the (variable?) nature of $\epsilon_0$ and $\mu_0$

In electromagnetism, the electric displacement field D represents the distribution of electric charges in a given medium resulting from the presence of an electric field E. Its relation to ...
Juan Moreno's user avatar
1 vote
1 answer
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When light passes from one medium to another. Depending on the density, it would experience a change in velocity. Why would its vector change as well?

For example, when light passes through water its velocity decreases and refraction occurs. Why? Why is there a change in direction. If I slow down a car I don’t suddenly turn left.
A new learner's user avatar
12 votes
5 answers
2k views

Could relativity be consistent if there are multiple light-like fields with different invariant speeds?

My understanding of real physical theory of electromagnetism goes like this: The Maxwell equations can be used to derive the speed of light; $$\nabla\cdot\textbf{E}=0$$ $$\nabla\cdot\textbf{B}=0$$ $$\...
spraff's user avatar
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Determining the time elapsed between two events in

I want to determine the time a photon needs in order to cover a distance, say $l_0$, where $l_0$ is the length of a spaceship (reference system S'). So, the photon is going from one end of the ...
schris38's user avatar
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Equivalence of speed and time flow

Is it right to assume that in 4-dimensional spacetime the speed of any object is constant? I mean a stationary object travels straight to the future but any spatial movement simply means that the ...
ElmoVanKielmo's user avatar
6 votes
2 answers
182 views

General relativity vs newtonian mechanics [duplicate]

My high school textbook briefly touched the topic of black holes, and this is how it defined them: "Consider a spherical body of mass $M$ and radius $R $. Suppose,due to some reason the volume ...
tensorman666's user avatar
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1 answer
51 views

Speed in a medium

Let's say, that there is medium, where the speed of light is $10^8 \, \rm m/s$. So in that medium if there exists a particle that is moving through without any hindrance what would be the speed limit ...
Le nerd's user avatar
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2 answers
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Phase difference in Michelson Interferometer

I'm currently taking a quantum mechanics course in university and our professor introduced us to the Michelson Interferometer (or at least what I believe is an apparatus based on it), where a light ...
JBatswani's user avatar
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1 answer
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Special relativity velocity addition formula precision [closed]

I tested the special relativity addition formula $$u_{\text{total}}=\frac{v+u}{1+\frac{vu}{c^2}}$$ and found that addition of small numbers converges to smaller speed, but bigger numbers to bigger ...
Vadim Ostanin's user avatar
1 vote
0 answers
69 views

Does squeezed light gravitate repulsively? [closed]

Since there are alternate regions of positive and negative energy densities in squeezed light, does that mean that the negative energy density parts gravitate repulsively? Since the stress energy ...
Peter's user avatar
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Can light speed change relative to me?

If a light-emitting object is stationary relative to me, then the wavelength of the light emitted would be normal. However, if the object is moving away from me, then the wavelength of the light would ...
VV_721's user avatar
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3 answers
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Faster than light is possible? [duplicate]

I know that some particles can go near or more than half the speed of light. So, say we shoot two particles in opposite directions at more than half the speed of light. Say one is moving at 50% the ...
user392759's user avatar

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